The present invention refers to an apparatus for feeding bulk material, in particular to a rotary feeder of the type including a housing provided with a top inlet and a bottom outlet and accommodating a cellular rotor which includes a plurality of radial vanes fixed on a shaft which is supported in lateral bearing covers.
In a rotary feeder of this type, the vanes of the rotor radially extend from the shaft to the inner wall surface of the housing and are confined in axial direction by circular lateral disk members. Bearing against the peripheral area of the lateral disk members at their end face which faces the adjacent bearing cover is a slide ring which is guided for axial displacement in the respective bearing cover in non-rotational manner. Both slide rings are made of polytetrafluoroethylene and seal chambers defined between the lateral disk members of the rotor and the respective bearing cover against penetration of dusty particles of the bulk material. Further, the slide rings prevent leakage of air through these chambers which would otherwise occur in view of the differential pressure between the inlet side and the outlet side of the rotary feeder.
The slide rings are urged against the lateral disk members by several prestressed helical compression springs which are supported by and uniformly spaced about the circumference of the respective bearing covers. Practice has shown that the frictional conditions and the forces exerted in the area of the sliding surfaces of slide ring and side disk are difficult to predict and to calculate in advance because of their dependency on numerous, partly time-varying parameters such as material of slide ring and side disk, pressure differential between inlet and outlet of the rotary feeder or wear condition of the slide ring. Further, the compression springs cause a local flexure of the slide rings in their contact area with the latter.
More importantly, however, an increasing wear of the slide rings results in a decreasing prestress of the compression springs and thus in a decrease of the contact pressure of the slide rings against the lateral disk members. Since for reasons of long service life of the slide rings e.g. or more than one year, it is generally desired to set the initial contact pressure at a level which does not exceed the pressure required for attaining a sufficient sealing action, this sealing action deteriorates with increasing wear during operation of the rotary feeder.
Although a decreasing prestress of the compression springs may be avoided through manual readjustment at regular e.g. monthly intervals, the need for additional readjusting mechanisms considerably complicates the structure of the rotary feeder and also increases maintenance thereof. Therefore, the contact pressure of the slide rings is frequently set at a higher level exceeding the level for optimum sealing when the rotary feeder comes from the production line. A higher contact pressure, however, results in increased wear and thus in shortened service life of the slide rings.